The goal is to study malignant lymphomas and leukemias using a combination of methodologies which will lead to better characterization and classification of these diseases. Cells isolated from 660 cases of non-Hodgkin's lymphomas, lymphocytic leukemias and reactive conditions have been studied during the past year using light microscopy, surface marker analysis, flow microfluorometry and chromosome analysis. (1) In one case of diffuse, well-differentiated lymphocytic lymphoma of 1gM/1gD, kappa class, an apparent second malignant lymphoma of large cell type of the lambda light chain class evolved. While well-differentiated lymphocytic lymphoma is known to transform into more aggressive large cell forms, this transformation has been shown by others to evolve from cells of the same light chain class. (2) In a patient with long standing CLL of the B-cell type, the development of cutaneous T-cell lymphoma (CTCL) was documented. The neoplastic lymphoid cells in the cutaneous infiltrate differed morphologically and immunologically from those in the peripheral blood. While a different lymphoproliferative disease was suspected based on morphology, immunological studies were required to document this difference. (3) Socalled "monocytoid" cells have characteristically been described in toxoplasmic lymphadenitis, although occasionally they can be seen in other diseases. Fresh frozen tissues from a series of lymph nodes showing morphological features suggestive of toxoplasmic lymphadenitis from nine patients were evaluated. Immunohistochemical studies revealed that monocytoid cells showed strong membrane staining for the presence of the B-1 and BA-1 antigens and expressed surface membrane associated immunoglobulin chains having a polyclonal immunostaining pattern. Monocytoid cells were devoid of cytoplasmic muramidase activity (CMA) and peanut agglutinin activity (PMA). Based on these findings, it is proposed that these cells be referred to as "monocytoid B lymphocytes."